Expanding systems of mountain roads in developing countries have significantly
increased the risk of landslides and sedimentation, and have created
vulnerabilities for residents and aquatic resources. We measured landslide
erosion along seven road segments in steep terrain in the upper Salween
River basin, Yunnan, China and estimated sediment delivery to channels.
Landslide erosion rates along the roads ranged from 2780 to 48 235 Mg ha
Although there is ample evidence of the effects of mountain road development on landslide initiation, only recently has this issue been raised within the context of sustainable development and the potential collapse of certain ecosystem functions (Sidle and Ziegler, 2012; Sidle et al., 2013). In particular, the extent of environmental damage caused by landslides along mountain roads in developing nations is poorly understood. While numerous international donors, non-governmental organizations (NGOs), and environmental advocates have attributed increased sedimentation in rivers and streams in these regions to shifting cultivation and deforestation (e.g., UNESCO, 1974; Eckholm, 1979; Volk et al., 1996; Marshall, 1999), more comprehensive investigations and analyses recognize that these land use practices exert much less influence on downstream sediment and aquatic resources than poorly constructed mountain roads (Sidle et al., 2006, 2007, 2011; Wasson et al., 2008; Ziegler et al., 2009). Of course, road and trail systems are associated with shifting cultivation and deforestation, but more recent, rapid expansion of road networks in mountainous terrain of developing nations have been linked to transitions from shifting cultivation to more intense agriculture, increased tourism, economic development, national defense, emergency evacuation routes, and hydropower development (Krongkaew, 2004; Nyaupane et al., 2006; Ziegler et al., 2012; Urban et al., 2013). Rural road development in Southeast Asia has been aggressively supported by organizations such as the Asian Development Bank, the Food and Agricultural Organization of the United Nations, and the World Bank, largely based on perceived socioeconomic benefits (van de Walle, 2002; Balisacan, 2005; Hettige, 2006). In most cases, long-term sustainability assessments that weigh the relative benefits and impacts of rural mountain roads, including socioeconomic tradeoffs, have not been conducted (Sidle et al., 2013).
Any road constructed in steep terrain will decrease the stability of the hillslope (Sidle and Ochiai, 2006). Roads cut into steep slopes promote landslides by removing upslope support and oversteepening the cut slope (Megahan et al., 1978; Rice, 1999; Sidle et al., 2006). Fill slopes, particularly when excavated fill material is poorly compacted, are susceptible to failure due to overloading and oversteepening of the slope, as well as when road drainage concentrates on these sites (Burroughs et al., 1976; Douglas et al., 1999; Sidle et al., 2011). As such, mid-slope mountain roads tend to create the most severe landslide problems because they experience both cut-and-fill failures including intercepting substantial quantities of subsurface water which often triggers landslides in fill slopes (Wemple et al., 2001; Sidle and Ochiai, 2006). If the terrain below these mid-slope roads is steep, much of the landslide material can directly reach streams or rivers, contributing significantly to sedimentation (Mills, 1997; Sidle et al., 2011). Because a significant portion of new rural road construction in northern Yunnan, China, as well as throughout developing regions of Southeast and East Asia, is occurring in steeply dissected mountainous regions, the associated landslides and sedimentation are problematic (Nyaupane et al., 2006; Wasson et al., 2008; Sidle and Ziegler, 2012). However, such land management effects have not been well documented, particularly with regard to sediment sources and delivery. A recent empirical assessment of the combined effects of structural control measures, vegetation restoration, and climate change on reduction of suspended sediment in the Kejie catchment within Yunnan noted that the former two practices were most effective in reducing suspended sediment (Ma et al., 2014); however, road inputs were not assessed in this study and the model used in the analysis (SWAT) has been clearly shown to ignore mass wasting contributions and produces erroneous sedimentation estimates in unstable terrain (Sidle, 2006).
Structural measures to prevent landslides along roads have been used effectively in vulnerable sites (e.g., Holtz and Schuster, 1996) but are prohibitively expensive in remote regions of most developing countries (Sidle and Ochiai, 2006). Check dams in streams and rivers can effectively remove sediment (Ma et al., 2014), but are expensive and do not address problems at the source or in upstream reaches, and are not sustainable control measures. Furthermore, little attention has been paid to road location and construction techniques in mountainous Southeast Asia (Sidle et al., 2004, 2006, 2011; Ziegler et al., 2004, 2012). As such, landslide issues need to be more carefully considered by many of the agencies that initiate and control the construction of such corridors in developing nations, as well as environmental groups and international organizations, which are focusing more on widespread land cover changes and hydropower development (Sidle and Ochiai, 2006; Tullos et al., 2013).
One of the first instances of heightened environmental awareness of
road-related landslides was associated with sedimentation of streams and
resultant impacts on fish habitat in the Pacific Northwest, USA during the
1970s and 1980s. Rates of landslide erosion from secondary forest roads in
unstable terrain of Oregon and Washington ranged from 25 to 155 Mg ha
Little emphasis has been placed on the impacts of landslides on
environmental health and human welfare in developing countries of Asia where
secondary mountain road systems are expanding at a rapid pace (Haigh, 1984;
Sakakibara et al., 2004; Castella et al., 2005). Within China, the total
road length of rural transportation networks increased by 5.5-fold during
the 30-year period from 1978 through the end of 2007 (China Road Construction
Report, 2008). A recent study (Sidle et al., 2011) that reported extremely
high levels of landslide erosion (1410–33,450 Mg ha
The steep Hengduan Mountains of western Yunnan Province are currently experiencing rapid development pressures due to the opening of access to remote villages, hydropower development, agriculture, tourism, forest exploitation, and other related aspects of economic development. This area includes the north–south trending, deeply dissected gorges of the “three great rivers” (i.e., Salween, Mekong, and Jinsha rivers) within “The Three Parallel Rivers of Yunnan Protected Areas”, inscribed by UNESCO on the World Heritage List in 2003 based on their unique geological history, geomorphic features, ecological processes, and rich biodiversity (UNESCO, 2003). The Salween River (known as the Nujiang in China) originates in the Tibetan Plateau and winds down steep gorges in northern Yunnan, along the border of southern Myanmar and northwestern Thailand, and eventually discharges into the Andaman Sea off the coast of Myanmar some 2800 km downriver. In our study region of northwestern Yunnan, the Salween River follows a major seismic fault. While considered to be one of the poorest regions in China (Su et al., 2012), the rapid anthropogenic change in this area is causing numerous impacts on the landscape and river systems.
The seven road segments we surveyed for landslide erosion are located in
five general locations along the Salween River in a region spanning about
120 km from 26
Elevations in the general study area range from about 800 to
Map showing the general locations of the road survey segments within the Salween River basin. Given the recent construction of these unpaved mountain roads, no road network map is available. The map in the upper left corner shows the general location of the study area within China and the greater Asian region.
Geographic information and landslide erosion data for the seven road segments surveyed in northwestern Yunnan.
Terrain that is undisturbed experiences limited landslide activity as does land cultivated by traditional agricultural practices. Only in the higher elevations on very steep slopes do significant numbers of surficial mass movements occur in relatively unmanaged terrain; these areas are typically remote from active channels. No major earthquakes occurred in the 5-year period prior to our field survey. Because of heavy seasonal rainfall, the steep nature of the terrain, the proximity of hillslopes to the Salween River (or its tributaries), and the highly altered bedrock, secondary roads cut into these hillsides are highly unstable. Whilst the types of igneous, metamorphic, and sedimentary rocks vary somewhat among and within the different road sites, the dominant feature that characterized bedrock failures is the highly fractured nature of the exposed rock. Based on our field experience, soil development over bedrock varies in depth from a few decimeters to several meters in some cases, depending on slope position and microtopography.
This part of China is under the influence of the Indian monsoon, and
described as a “warm-dry climate”, being a combination of subtropical and
alpine climates. Annual mean temperature (average from 1961 to 2010) is
20.2
Based on our field observations, current development in the region appears
to be paying little to no attention to road location or construction methods
related to the control of mass wasting. Secondary mountain roads are
typically constructed by hydraulic shovels, large back hoes, or
indiscriminate blasting, and the excavated material is simply disposed onto
the side slopes just below the road. The hillsides are characteristically
very steep (
The highest landslide erosion occurred along a 1-year old road (DXD) leading to a remote village and a future hydropower plant just south of Daxingdi, Yunnan.
In June 2010, landslides and related sediment delivery to stream and river channels were assessed along seven road segments within the Salween River basin in northwestern Yunnan, China (Table 1). Lengths and widths of landslides were measured with metric tapes where possible or with a laser distance meter (range finder) when slopes were too dangerous to traverse. Depths around the flanks of landslides on cut and fill slopes were measured directly where possible and otherwise visually estimated to facilitate calculation of landslide volumes. For some of the cut slope failures, it was clear that the entire landslide mass was trapped on the road; thus, dimensions of the landslide deposit were measured instead of the failure area. Numbers of landslides in cut and fill slopes were adjusted for the age of the road and reported as number per kilometer of road per year.
The calculated landslide volumes were converted to units of mass using an
assumed conservative bulk density of 1.3 Mg m
To estimate sediment delivery from road-related landslides, we examined the steepness and uniformity (breaks vs. no breaks) of the slope below each slide, as well as any evidence of deposition and the connectivity with the channel. While our visual estimates are somewhat crude approximations of sediment delivery, they are based on geomorphic attributes that control and affect sediment fluxes. At one site (GXK), the channel was relatively distant from the road, thus no connectivity (or sediment delivery) was noted. This does not mean that the sediment would never reach a stream; rather it was deposited on the landscape and could be later entrained by surficial processes, similar to surface erosion from more widespread land use in the region (e.g., Sidle et al., 2006). As such, sediment delivery estimates from roads are conservative. Nevertheless, the direct connectivity of road-related landslides with channels proved to be the most efficient and prodigious conduit of sediment delivery.
A total of 312 landslides were measured along about 5 km of unimproved roads
at the seven road survey segments in the Salween River valley. Rates of
landslide erosion at all seven road survey segments are extremely high by
all standards and comparisons (Table 1). The highest rate of landslide
erosion (48 235 Mg ha
Aside from the surveyed segment of the village road near Ganxiangke (GXK),
which was remote from streams, the delivery of landslide sediment to the
Salween River and its tributaries was
Road leading to a mountain village on the west side of the Salween River about 45 km south of Fugong. Upper portion of the road had (SFG1) the lowest landslide erosion of all surveyed segments, while the lower portion of the road (SFG2) had fewer numbers of, but larger, cut slope failures. Landslide erosion rates were significantly higher at SFG2 compared to SFG1 – 3.5 times higher for fill slope and 4.4 times higher for cut slope failures. The access bridge across the river is in the lower right corner of the photo.
The two widest roads (DXD and FG1) had the highest levels of landslide erosion. Wider roads cut into steep terrain disturb a much greater area than narrower roads and tend to destabilize hillslopes to a greater extent (Megahan, 1977; Sidle et al., 1985). Given that all of the roads examined were relatively new (or recently widened), erosion rates reported herein may be higher than longer-term averages. Nevertheless, temporal trends in erosion rates along these mountain roads are complicated by the frequent obliteration and extensive blockage of roads during large storm events (including landslides). Such major disturbances either require new road construction or extensive road widening and excavation, thus perpetuating the cycle of active landsliding.
Comparison of numbers and mean mass of landslides along cut and fill slopes of the seven monitored road segments.
High rates of landslide sediment delivery to the Salween River
from the
Landslide erosion from cut slopes and fill slopes for the seven surveyed road segments.
Size distribution for all surveyed landslides from
The relative proportion of landslide erosion along cut slopes and fill slopes
varied among sites and was strongly influenced by terrain characteristics,
the depth of road cuts into the hillside, and the disposition of fill
material. Overall, more landslide sediment was produced from fill slopes
(38 170 Mg) compared to cut slopes (29 055 Mg) even though cut slope failures
outnumbered fill failures by 235 to 77 (more than a 3 : 1 ratio). Three
examples stand out as having disproportionately higher cut slope or fill slope
landslide erosion. The DXD road had a large number (
The distribution of all cut slope landslides (
Overall the mean mass of individual fill slope landslides was 4 times higher than cut slope slides; an exception to this trend was FG1 with the five very large cut slope landslides. The widest difference between mean fill and cut slope landslide mass was at DXD with a ratio of 11.6 (Table 2). DXD also had the largest mean mass of fill slope failures, partially attributable to the long, steep, and uniform slopes below the road (Fig. 4a). The sites with the smallest mean landslide masses along cut slopes were SFG1 and WTW (Table 2).
Overall there were about 3 times more cut slope compared to fill slope landslides, and cut slope landslides were more frequent than fill slope failures at all seven road segments (Table 2). SFG1 had the largest number of cut slope failures, but, as noted, these were small and constituted the second smallest landslide erosion rate of all seven road segments; DXD had a rather large number of sizable cut slope failures. WTW and GXK both had high numbers of landslides along cut slopes, but with small to intermediate mean masses (Table 2). Both DXD and WTW had the largest numbers of fill slope failures together with the highest sediment delivery estimates (Tables 1 and 2) – these sites were both proximate to a tributary and the main stem of the Salween River, respectively. Few fill slope failures occurred in FG1 and FG2, and especially in GXK. GXK was situated away from the river and the slope below the road was gentle, containing rice paddy fields.
As a solution to the landslide and associated environmental damages caused by inadequate attention to secondary road location and construction practices in this region, we propose a more sustainable approach that assesses not only the perceived social and economic benefits of these roads, but also the long-term environmental and human welfare impacts. Many of the new roads are inoperable during the rainy season or required extensive excavation or maintenance to remain open (Figs. 2 and 7a). In the worst cases, partially completed roads were abandoned because of persistent landslides leaving a legacy of sedimentation problems with no socioeconomic benefits whatsoever (Fig. 7b). The prolific road-related landslides and associated riverine sedimentation that is occurring within the Salween River basin could push portions of this ecosystem to tipping points where thresholds are breached, causing the collapse of certain ecosystem processes and functions (Sidle et al., 2013). Impacts that could occur in the foreseeable future include (1) extensive areas of degraded site productivity and altered vegetation due to landsliding; (2) degraded downstream water quality and aquatic habitat; (3) transport of contaminated sediments downstream; (4) alteration of the morphology of tributary streams and the main stem of the Salween River; (5) catastrophic debris flows in sediment-laden tributaries; (6) increased flood potential due to reduced channel transmission capacity; and (7) impacts on livelihoods and economies of water users in communities downstream. Some of these effects are already being realized in this region as documented in a nearby tributary of the Mekong River in Yunnan (Sidle et al., 2011).
Decision framework for sustainability assessment of mountainous terrain in northwestern Yunnan, China, where extensive road construction is being proposed (modified extensively from Sidle et al., 2013).
Moving forward, there is an urgent need to develop a systems-based approach for more sustainable mountain road development before tipping points are reached in these ecosystems. In northwestern Yunnan, this would include detailed landslide hazard assessments prior to road planning and construction activities. Ideally, such analyses should identify the probability of exceeding landslide trigger thresholds (in this area, largely rainfall) coupled with estimates of decreased slope stability associated with different road locations and construction techniques (Fig. 8). This approach would allow for tradeoffs between socioeconomic development and long-term environmental and human welfare impacts by articulating acceptable levels of landslide erosion, with an eye towards avoiding tipping points where site productivity, human welfare, ecological attributes, flooding, and aquatic habit are not compromised in the long term. Tradeoffs for road development could include alternative locations and construction techniques, assessing “storm-proofing” roads vs. continual widening and maintenance, considering multiple road uses, incorporating climate change impacts, and reevaluating the necessity of the road. In particular, road location strategies can go a long way towards ameliorating landslide problems; these include (1) optimizing the expected lifetime of the road with uses; (2) ensuring a balance of minimizing road length and minimizing steep gradients; (3) avoiding deep cuts into unstable substrate (especially bedrock dipping parallel to the hillslope); (4) utilizing valley bottom and ridge-top roads whenever feasible; (5) avoiding seasonally wet areas (e.g., hollows); (6) reducing the width of mid-slope roads; (7) avoiding crossing old landslides, particularly undercutting the toe or loading the head of dormant failures; (8) rolling roads to fit hillslope contours and across drainage culverts; and (9) minimizing stream crossings (Megahan et al., 1978; Sidle and Ochiai, 2006). Furthermore, planting deep rooted woody vegetation on road fills can enhance long-term stability (e.g., Stokes et al., 2009). Such a systems-based approach that considers all possible road uses and benefits against environmental and human welfare costs (Fig. 8) offers a much more robust and sustainable alternative.
Multi-criteria decision analysis has been applied to similar ecosystem sustainability challenges in which cost/benefit tradeoffs need to be assessed jointly among environmental, social, and economic objectives (e.g., Linkov et al., 2006; Macleod et al., 2007). Throughout much of mountainous Southeast Asia, benefits associated with secondary road development that need quantification include (1) opening economic markets for goods and services produced in remote mountain villages; (2) tourism opportunities; (3) access to hydroelectric generation facilities; (4) educational opportunities; (5) emergency evacuation; and (6) defense of national borders. Cost assessment associated with mountain road development should focus on (1) environmental impacts of hydroelectric power facilities, increased tourism, and forest exploitation; (2) direct impacts of road-related landslides on settlements, sediment loads in rivers, water quality, and aquatic habitat; (3) loss of site productivity on hillslopes affected by landslides; (4) impacts of relatively clean and contaminated sediments in downstream water supplies; (5) effects on channel conveyance, flooding and potential debris flows; (6) siltation of existing reservoirs; and (7) environmental consequences of unintended forest exploitation (Fig. 8).
Successful implementation of multi-criteria decision analyses related to road development and associated environmental and natural resources planning in this region will require the engagement of diverse stakeholders with government planning agencies, donor organizations, and science experts. An important aspect of this analysis is the need to incorporate landslide risk associated with extreme events – i.e., storms or other triggers like earthquakes. This will require better spatial and temporal coverage of precipitation in the region. Furthermore, some of the consequences associated with ecosystem tipping points (e.g., floods, debris flows, vegetation changes, aquatic habitat degradation) require a probabilistic approach to assessment of risk. Ecosystem goods and services as well as environmental costs should be appropriately valued; in cases where environmental resources have no apparent market value, alternative techniques can be used (Gregory, 2000; Ananda and Herath, 2009). Inherent to the success of such a decision analysis is the concurrent engagement of government planning agencies that deal with road construction, river management, catchment management (including land use), aquatic habitat and biodiversity, and economic development to resolve interagency conflicts and consider relevant stakeholder opinions together with scientific expertise and evidence (e.g., Macleod et al., 2007).
Our investigations of landslide erosion along seven different mountain road
segments in the upper Salween River basin confirm findings from a prior
study (Sidle et al., 2011) in a single tributary of the Mekong River near
Weixi, Yunnan. The erosion rates measured along these seven unpaved mountain
road segments (2780 to 48 238 Mg ha
At all sites, landslides on cut slopes were more numerous, but
characteristically smaller than on fill slopes. Where hillslopes were very
steep below the excavated road, landslide erosion from fill slopes was
greater than from cut slopes. Although few instances of very large landslides
(
With the high level of road-related landslide sediment already being transported into the Salween River, a more sustainable approach is needed to assess future road system development. A decision tool is needed that includes a rather detailed analysis of landslide susceptibility based on how thresholds for landslide triggers (i.e., rain events) would be modified by various road locations and different construction techniques. In this systems-based analysis, one could assess tradeoffs amongst socioeconomic benefits of road networks against costs of protecting long-term human welfare, environmental attributes, and site productivity. Multi-criteria decision analysis could be employed to properly assess the road-related sediment issue in the context of alternative practices and other land uses. This systems-based approach needs to be embraced by local governments, environmental groups, NGOs, and international organizations and donors, who seem to be focusing almost exclusively on the socioeconomic benefits of roads in this developing mountainous region. Countries located downstream of China within the Salween River basin (Myanmar and Thailand), as well as the other two major river basins in Yunnan (Mekong and Jinsha rivers – Thailand, Cambodia, Laos, and Vietnam), need a sufficient supply of clean water to support livelihoods and development. Trans-boundary sediment issues associated with recent road construction in Yunnan pose serious problems for downstream users. Clearly, a paradigm shift is needed to embrace the concepts of sustainability in conjunction with road development in northwestern Yunnan, as well as in other potentially unstable mountain environments.
This project was supported by funding from the French National Institute for Agricultural Research (INRA, Jeune Equipe) and the BMU (Germany) International Climate Initiative funded project “Ecosystems Protecting Infrastructure and Communities” (EPIC, coordinated by IUCN and ProAct, Switzerland). Gratitude is expressed to K.-F. Cao (XTBG and the University of Guangxi, China), W. Ma (Kunming Institute of Botany, China) and F.-X. Mine (ISARA, France), for assistance in field investigations and, logistics and to Lourdes Prieto for preparing the site map. This document has been reviewed in accordance with US Environmental Protection Agency policy and approved for publication. The opinions presented herein are those of the authors and do not represent US EPA, ONCFS, INRA, or University of the Sunshine Coast. Edited by: K. Chang Reviewed by: four anonymous referees